feat(ipa): add Home Manager config for IPA primary user

Any enrolled host now automatically gets a Home Manager profile for the
IPA primary user (vars.ipaUser = "wayne"), covering what IPA doesn't:
dotfiles, user-scoped packages (tmux, sshfs), and EDITOR variable.

The home directory is pre-created by systemd-tmpfiles so HM activation
succeeds on steady-state systems before first login; pam_mkhomedir remains
as a fallback for fresh deploys where SSSD hasn't cached the user yet.

A minimal users.users stub satisfies NixOS's assertion requirements
(isNormalUser + group) that arise because home-manager.useUserPackages
creates a users.users entry to install packages to /etc/profiles/per-user/.
The stub is shadowed by SSSD at runtime (security.ipa sets passwd: sss files).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

variables.nix

@@ -80,6 +80,11 @@
   # one-line change.
   primaryUser = "nixos";
 
+  # Primary IPA/domain user. Home Manager is configured for this user on every
+  # IPA-enrolled host (see modules/ipa/client.nix) to manage the environment
+  # that IPA itself doesn't cover: dotfiles, user packages, session variables.
+  ipaUser = "wayne";
+
   # HA file server cluster
   # LAN IPs (vmbr0 / ens18) — client-facing: iSCSI initiators, NFS, management.
   # Storage IPs (vmbr1 / ens19) — isolated internal bridge, used for DRBD